LA-ICP-MS zircon U-Pb dating of Ordovician Mengmao monzogranite in Longling area of western Yunnan Province and its tectonic setting

Mengmao monzogranite, exposed in Longling area of western Yunnan, was geochemically analyzed and discussed to probe into its genesis, ore-forming material source and tectonic setting. The results show that Mengmao granite has high SiO2(72.17%~76.45%) and Al2O3, with A/CNK ratios ranging between 0.86 and 1.45 (1.12 on average). These data suggest that Mengmao granite belongs to typical peraluminous granite, with over-saturated Si and Al . The ∑REE values range from 138.17×10-6 to 383.68×10-6, and the REE patterns show right oblique feature and obvious negative Eu anomaly. Trace elements show enrichment of LILE (large-ion lithophile elements) such as K, Sr, Rb and Ba and depletion of HFSE (high field strength elements) such as Zr, Ta, Nb and Th. Magmatic source was typical of crustal genesis and was probably formed by rising and emplacement of granitic magma, which had been partially melted by greywacke. LA-ICP-MS U-Pb dating of zircons indicates that Mengmao granite was emplaced during the Middle Ordovician (454.7±1.5Ma), with post-collisional magmatic activity features. Mengmao granite, as post-collisional granite and a product of magmatism by late Gondwana land orogenic movement, was formed in the tectonic transition stage from syn-collision (extrusion environment) to post-collision (stretching environment). The result of the researches on Mengmao monzogranite has significance in revealing tectonic attributes of Nujiang fault belt in western Yunnan.